Synthesis and binding properties of cyclohexapeptide somatostatin analogs containing naphthylalanine and arylalkyl peptoid residues

Peptides and proteins as a continuing exciting source of inspiration for peptidomimetics

Despite their enormous diversity in biological function and structure, peptides and proteins are endowed with properties that have induced and stimulated the development of peptidomimetics. Clearly, peptides can be considered as the "stem" of a phylogenetic molecular development tree from which branches of oligomeric peptidomimetics such as peptoids, peptidosulfonamides, urea peptidomimetics, as well as β-peptides have sprouted. It is still a challenge to efficiently synthesize these oligomeric species, and study their structural and biological properties. Combining peptides and peptidomimetics led to the emergence of peptide-peptidomimetic hybrids in which one or more (proteinogenic) amino acid residues have been replaced with these mimetic residues. In scan-like approaches, the influence of these replacements on biological activity can then be studied, to evaluate to what extent a peptide can be transformed into a peptidomimetic structure while maintaining, or even improving, its biological properties. A central issue, especially with the smaller peptides, is the lack of secondary structure. Important approaches to control secondary structure include the introduction of α,α-disubstituted amino acids, or (di)peptidomimetic structures such as the Freidinger lactam. Apart from intra-amino acid constraints, inter-amino acid constraints for formation of a diversity of cyclic peptides have shaped a thick branch. Apart from the classical disulfide bridges, the repertoire has been extended to include sulfide and triazole bridges as well as the single-, double- and even triple-bond replacements, accessible by the extremely versatile ring-closing alkene/alkyne metathesis approaches. The latter approach is now the method of choice for the secondary structure that presents the greatest challenge for structural stabilization: the α-helix.

Synthesis and biologic activity of conformationally constrained analogs of L-363,301

We report the synthesis, biological activity and conformational analysis of analogs of the cyclic hexapeptide L-363,301, c[Pro6-Phe7-D-Trp8-Lys9-Thr10-Phe11] (numbering as in the native hormone somatostatin-14). The d-Trp in position 8 was replaced with (2R,3S)- and (2R,3R)-beta-MeTrp respectively, with an added methyl group in the beta position of Trp. The objective of our study was to determine the potency and selectivity generated by the added constraint in the beta position of the d-Trp upon binding to human somatostatin receptors hsst1-5. We synthesized the building blocks enantioselectively and incorporated them into the peptides by SPPS. Competition binding assays revealed that both compounds 2 and 3 were selective for hsst2 over hsst5. The (2R,3S) analog 2 was approximately 30 times more potent at hsst2 than the (2R,3R) analog 3. Interestingly, the (2R,3R) compound showed no binding affinity at hsst5.

Somatostatin analogs

Somatostatin is a hypothalamic peptide hormone that inhibits the secretion of growth hormone, glucagon, insulin, gastrin and secretin, and also plays a role in neural transmission. Because of its wide range of possible clinical applications hundreds of somatostatin analogs have been synthesized and bioassayed to date. This review gives a historical perspective, summarizing approximately 30 years of research on somatostatin. The main focus is on the structure-activity relationships and conformational studies of the last generation of somatostatin agonists and their selectivity for five somatostatin receptor subtypes. Achievements in the synthesis of nonpeptide somatostatin analogs, as well as the first somatostatin antagonists, are also discussed. Finally, the use of a cyclic somatostatin scaffold to design ligands for other G-protein-coupled receptors, such as opioid and melanocortin receptors, is mentioned.

Conformational analyses of cyclic hexapeptide analogs of somatostatin containing arylalkyl peptoid and naphthylalanine residues

We report the conformational analysis by 1H-NMR in DMSO and computer simulations involving distance geometry and molecular dynamics simulations of peptoid analogs of the cyclic hexapeptide c-[Phe11-Pro6-Phe7-D-Trp8-Lys9-Thr10] L-363,301 (the numbering refers to the positions in native somatostatin). The compounds c-[Phe11-Nphe6-Nal7-D-Trp8-Lys9-Thr10] (Nphe6-Nal7 analog 1), c-[Nal11-Nphe6-Phe7-D-Trp8-Lys9-Thr10] (Nal11-Nphe6 analog 2) and c-[Phe11-Nnal6-Phe7-D-Trp8-Lys9-Thr10] (Nnal6 analog 3), where Nphe denotes N-benzylglycine and Nnal denotes N-(1-naphthylmethyl)glycine, are subjected to SAR studies in order to investigate the influence of the bulky naphthyl aromatic ring on the conformation. The Nal11-Nphe6 and Nphe6-Nal7 analogs exhibit potent binding to the hsst2, hsst3 and hsst5 receptors, whereas the Nnal6 analog has decreased binding affinity to all receptors but is more selective towards the hsst2 than the other two analogs and L-363,301. The conformational search employing distance geometry, energy minimization and molecular dynamic simulations gives insight into the conformational flexibility of these analogs. The molecules adopt both cis and trans orientations of the peptide bond between residues 11 and 6. The cis isomers of these analogs adopt type II' beta-turns with D-Trp in the i + 1 position and type VIalpha beta-turns with the cis peptide bond between residues 6 and 11. The results of free and distance restrained molecular dynamics simulations at 300 K indicate that the Nphe6-Nal7 and Nal11-Nphe6 compounds adopt a preferred backbone conformation which can be described as 'folded' about residues 7 and 10. The Nnal6 analog, which binds less effectively to the hsst receptors, has a more flexible backbone structure than the Nal11-Nphe6 and Nphe6-Nal7 analogs and prefers a 'flat' structure with regard to the orientations about Phe7 and Thr10 during molecular dynamics simulations.